electrolytic silver plating solution

ABSTRACT

A silver electroplating solution containing a cyanide as a silver source, said silver electroplating solution characterized by containing at least one type of compound of As, Tl, Se, and Te as a brightener and a brightness adjuster having a benzothiazole skeleton or benzoxazole skeleton. This plating solution utilizes the high speed property of gloss agents to the maximum, does not have an effect on the current density, gives stable non- or semi-gloss plating appearance, and facilitates control.

TECHNICAL FIELD

The present invention relates to a silver electroplating solution, moreparticularly relates-to a high speed non-gloss or high speed semi-glosssilver electroplating solution using a cyanide as a silver source.

BACKGROUND ART

In the past, for silver electroplating solutions, silver electroplatingsolutions using a cyanide as a silver source and including compounds ofAS, Tl, Se, and Te as brighteners have been known (Japanese Patent No.2756300).

By adding these gloss agents, there are the advantages that glossysilver plating films are obtained even with slight amounts of addition,the brightness degree increases the greater the amount of addition, theworking current density is raised, and the speed is increased.

On the other hand, when forming silver plating films at front ends ofinner leads or die pads etc. of lead frames for semiconductor packages,if bright (meaning becoming close to mirror surfaces), converselyproblems such as image recognition not being possible at the time ofwire bonding and sealability becoming inferior arise. Consequently,plating aiming at matt or semi-bright has been performed. Therefore, ithas been necessary to lower the amounts of addition of brighteners.

In the case of the above gloss agents, however, originally only verysmall amounts were added, so if further lowering the amounts ofaddition, analysis becomes difficult and routine control of the amountsof addition by analysis becomes impossible. In practice, the amounts ofaddition are judged by the experience and judgment of the workers. Goodcontrol over the amounts of addition is therefore difficult.

Further, if reducing the amounts of addition of gloss agents, theworking current density becomes low and the range becomes narrower, theproduction efficiency drops, and plating defects easily occur. In thiscase, if raising the working current density to raise the productionefficiency, so-called “burnt deposits” or uneven plating ends upoccurring.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a silver electroplatingsolution using a cyanide as a silver source which utilizes the highspeed property of brighteners to the maximum, does not have an effect onthe current density, gives a stable matt or semi-bright platingappearance, and facilitates control.

The silver electroplating solution using a cyanide as a silver source ofthe present invention is characterized by containing at least one typeof compound of As, Tl, Se, and Te as a brightener and a brightnessadjuster having a benzothiazole skeleton or benzoxazole skeleton.

The solution should contain as a metal salt 50 to 300 g/liter of asilver-like alkali metal cyanide and as a conductive salt 50 to 300g/liter of at least one of a citrate, phosphate, tartarate, andsuccinate. These conductive salts function also as pH buffers. Further,as a buffer, it is also possible to add boric acid or a borate.

Further, in accordance with need, it is also possible to add asurfactant. As such a surfactant, a nonionic surfactant having apolyoxyethylene chain or a fluorine-based surfactant is suitable. Thesesurfactants should be added in amounts of 0.001 to 10 g/liter or so.

As the brighteners, in the same way as the past, compounds of As, Tl,Se, and Te are used.

As these brighteners, potassium arsenite, sodium arsenite, potassiumsulfate, thallium formate, selenium dioxide, potassium selenocyanate,telluric acid, tellurium dioxide, etc. are suitable.

By using these compounds for gloss agents, the brightness degree may beadjusted, but control of fine amounts in the matt to semi-bright rangeis not easy as explained above.

In the present invention, by adding the later mentioned brightnessadjuster to the plating solution, the brightness degree can be keptdown. Therefore, the amount of brightener can be managed by an analysissystem (for example, ICP analysis or atomic absorption analysis).Further, the amount of brightener can be increased. Therefore, theworking current density can be raised. The range becomes broader andtherefore the production efficiency can be improved and the workefficiency can be improved.

The amount of addition of the brightener is suitably in the range of0.005 to 50 mg/liter, optimally the range of 0.01 to 5 mg/liter. Thisrange of amount of addition is the normal range of a brightelectroplating (bright silver plating) solution.

The amount of addition of the gloss adjuster is suitably in the range of1 to 1000 mg/liter, optimally the range of 10 to 100 mg/liter. If theamount of addition of the brightness adjuster is less than 1 mg/liter,there is little brightness adjustment effect and the working currentdensity also will not be able to be raised. Even if more than 1000mg/liter, the effect will not change that much compared with the amountof addition.

Further, the amount of addition of the brightness adjuster is suitablychanged in accordance with the amount of the brightener.

For example, when the amount of addition of the brightener is about 0.05to 0.1 mg/liter, the amount of the brightness adjuster is suitably madearound 10 mg/liter, while when the amount of addition of the brighteneris about 0.5 mg/liter, the amount of the brightness adjuster is suitablymade around 30 mg/liter.

As the compound having a benzothiazole skeleton of the brightnessadjuster, the following compounds are suitable:

-   -   R: H, CH₃, CH₃O    -   T: H    -   V: H, SO₃Na    -   Y: H, CH₃    -   X: one of the following a to i

That is, in the above, R is H, CH₃, or CH₃O, T is H, V is H or SO₃Na, Yis H or CH₃, and X is any of a to i. Further, as the compound having abenzoxazole skeleton of the gloss adjuster, the following compound issuitable:

-   -   X: —CH₂CH₂CH ₂SO₃Na    -   Y: one of the following a to b

That is, in the above, X is —CH₂CH₂CH₂SO₃Na and Y is any of the above aor b.

BEST MODE FOR CARRYING OUT THE INVENTION

The compositions of plating solutions of Examples 1 to 5 and ComparativeExamples 1 to 2 are shown in Table 1. TABLE 1 Comp. Comp. Ex. 1 Ex. 2Ex. 3 Ex. 4 Ex. 5 Ex. 1 Ex. 2 KAg(CN)₂ 80 80 80 60 100 80 100 K₂C₄H₄O₆80 — — — — — — K₃C₆H₅O₇ — 100 70 20 70 — 100 K₂HPO₄ — — 30 80 90 —K₄P₂O₇ — — — — 30 — — H₃BO₃ 20 20 — — 20 — — KSeCN 0.0005 0.001 0.0010.0005 0.0001 — 0.0005 C₂₀H₂₆N₄O₆S₂ 0.1 — — — — — — C₂₁H₁₄N₃NaO₃S₃ —0.001 — — 0.01 — — C₂₆H₃₂N₃NaO₆S₂ — — 0.01 — — — — C₂₈H₁₉N₅Na₂O₆S₄ — — —0.1 — — — C₁₂H₂₅(CH₂CH₂O)₂₃H 0.01 — 0.01 — — — H(OCH₂CH₂)_(n)OH — 0.1 —1 0.1 0.1

Note that the names of the compounds of Table 1 are shown in Table 2.TABLE 2 Chemical structure Chemical name KAg(CN)₂ Silver potassiumcyanide K₂C₄H₄O₆ Potassium tartarate K₃C₆H₅O₇ Tripotassium citrateK₂HPO₄ Dipotassium hydrogenphosphate K₄P₂O₇ Potassium pyrophosphateH₃BO₃ Boric acid KSeCN Potassium selenocyanate C₂₀H₂₆N₄O₆S₂ Basic Blue41 (Compound 1-i) C₂₁H₁₄N₃NaO₃S₃ Primulin (Compound 1-d) C₂₆H₃₂N₃NaO₆S₂Melocyanine 540 (Compound 1-c) C₂₈H₁₉N₅Na₂O₆S₄ Mimosa (Compound 1-c)C₁₂H₂₅(CH₂CH₂O)₂₃H Polyoxyethylene lauryl ether H(OCH₂CH₂)_(n)OHPolyethylene glycol

Table 3 shows the current density range, gloss degree, etc. TABLE 3 Goodcurrent density Brightness Uneven- Protrusions Die Die Heat range/A/dm²degree ness of silver bondability shareability resistance Ex. 1 50 to200 0.3 to 0.4 None None Good Good Good Ex. 2 50 to 200 0.3 to 0.4 NoneNone Good Good Good Ex. 3 50 to 200 0.3 to 0.4 None None Good Good GoodEx. 4 50 to 150 0.3 to 0.4 None None Good Good Good Ex. 5 50 to 300 0.3to 0.4 None None Good Good Good Comp. Ex. 1 50 to 90  0.1 to 0.3 NoneNone Good Good Good Comp. Ex. 2 50 to 300 0.4 to 1.2 None None Good GoodGood

Examples 1 to 3 show plating solutions using gloss adjusters havingbenzothiazole skeletons, while Examples 4 and 5 show ones changed insilver concentration. Comparative Example 1 shows a solution of a simplecomposition of just a silver salt and conductive salt, while ComparativeExample 2 shows a solution like in the past including a brightener andsurfactant.

The plating solutions of these compositions were used to conduct Hullcell tests and the gloss adjustment effects were evaluated from the Hullcell patterns.

Further, 3 cm×3 cm copper test pieces were silver plated to 1 cmφ and 5μm by jet plating and compared for current densities giving goodsemi-bright appearances.

EXAMPLES 1 to 3

The pH of the solutions were stable at 8 to 9. The Hull cell patternshad semi-bright appearances over broad ranges. The platings were good inappearance with even brightness degree and no unevenness. The platingfilms prepared by jet plating tests had semi-bright appearances withbrightness degrees of 0.4±0.05 or no substantial variation in thecurrent density range of 50 to 200 A/dm². Further, the crystals of thesilver film having matt or a semi-bright were dense and the mountingproperties were sufficiently satisfactory.

Further, similar results were obtained when conducting similarexperiments for Direct Yellow 8, Thioflavin S, and Thioflavin T for thebrightness adjuster.

Note that the brightness degree was measured using a Densitometer ND-1made by Nippon Denshoku. Incidentally, a brightness degree of not morethan 0.2 is defined as a matt plating and a brightness degree of up to0.8 as semi-bright plating.

EXAMPLE 4

This composition has a lower silver concentration compared with Examples1 to 3. The semi-bright region of the Hull cell pattern had asemi-bright appearance of an even dull appearance. In a jet platingtest, no burnt deposits occurred up to a high current density range ofover 100 A/dm². The obtained plating film had an even semi-brightappearance.

EXAMPLE 5

This composition has an increased silver concentration compared withExamples 1 to 3. The semi-bright region of the Hull cell pattern wasbroad and an even, good film was obtained. Further, in a jet platingtest, a silver film having a semi-bright appearance was obtained up to arange of current density of 300 A/dm². The film was of uniform qualitywith no blistering or unevenness.

COMPARATIVE EXAMPLE 1

This composition contains no additives at all. The Hull cell pattern waswhite overall and the appearance was a white one close to a burn. In ajet plating test, burnt deposits ended up occurring with a currentdensity of over 100 A/dm². The current density giving a semi-brightappearance was a narrow range of 50 to 90 A/dm².

COMPARATIVE EXAMPLE 2

This composition contains a brightener and surfactant as in the past.The brightness degree of the semi-bright region of the Hull cell patternwas a somewhat shiny brightness. Further, a sample prepared by a jetplating test was free of burnt deposits up to a range of current densityof 200 A/dm² and was satisfactory in mounting properties, but theappearance became an appearance with somewhat of a brightness degree ofabout 0.8 to 1.0.

INDUSTRIAL APPLICABILITY

As explained above, the silver electroplating solution of the presentinvention using a cyanide as a silver source and containing a brightnessadjuster can give good matt to semi-bright silver plating film withlittle variation in brightness degree in a broad current density rangeand free of unevenness or burn with a high current density. Therefore,ring plating of difficult-to-plate SO type lead frames or QFP leadframes and high speed, stable plating of complicatedly shaped leadframes and other plated objects become possible. Further, the brightnessdegree of the silver plating films obtained from these baths can becontrolled in brightness effect of the brightener by adding a glossadjuster and the brightness degree of the silver plating film can befreely adjusted. Further, by raising the concentration of the brightnessadjuster, the effect of suppression of brightness becomes greater, soincreasing the concentration of the brightener becomes possible, controlof the concentration by analysis becomes possible, and a high workingcurrent density is obtained, so the production efficiency is improved.

1. A silver electroplating solution containing a cyanide as a silversource, said silver electroplating solution characterized by containing:at least one type of compound of As, Tl, Se, and Te as a brightener anda brightness adjuster having a benzothiazole skeleton or benzoxazoleskeleton.
 2. A silver electroplating solution as set forth in claim 1,characterized by containing said brightener in an amount of 0.005 to 50mg/liter.
 3. A silver electroplating solution as set forth in claim 1 or2, characterized by containing said brightness adjuster in an amount of1 to 1000 mg/liter.
 4. A silver electroplating solution as set forth inclaim 1 or 3, characterized by containing said brightener in an amountof 0.01 to 5 mg/liter.
 5. A silver electroplating solution as set forthin any one of claims 1 to 4, characterized by containing as a surfactanta nonionic surfactant having a polyoxyethylene chain or a fluorine-basedsurfactant in an amount of 0.001 to 10 g/liter.
 6. A silverelectroplating solution as set forth in any one of claims 1 to 5,characterized by containing as a metal salt 50 to 300 g/liter of asilver-like alkali metal cyanide and as a conductive salt 50 to 300g/liter of at least one of a citrate, phosphate, tartarate, andsuccinate.
 7. A silver electroplating solution as set forth in any oneof claims 1 to 6, characterized in that the compound having abenzothiazole skeleton is one of the following compounds:

R: H, CH₃, CH₃O T: H V: H, SO₃Na Y: H, CH₃ X: one of the following a toi


8. A silver electroplating solution as set forth in any one of claims 1to 7, characterized in that the compound having a benzoxazole skeletonis one of the following compounds:

X: —CH₂CH₂CH₂SO₃Na Y: one of the following a to b